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Subject: Re: [PATCH 09/10] fortran: Support clobbering of variable
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To: Thomas Koenig <tkoenig@netcologne.de>, Harald Anlauf <anlauf@gmx.de>,
 Richard Biener <richard.guenther@gmail.com>
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Le 21/09/2022 à 11:57, Thomas Koenig a écrit :
> 
> Hi Harald,
> 
>> I think I understand much of what is said, but I feel that I do
>> not really understand what *clobber* means for the different
>> beasts we are discussing (although I have an impression of what
>> it means for a scalar object).
> 
> Obviously, "clobber" means taking a big stick and hitting the beast
> in question over the head with it :-)
> 
> More seriously: My understanding of a clobber it is a hint to
> the middle end that the value in question will not be used,
> and that operations leading to this value can be removed,
> unless they are used otherwise.
> 
My understanding is that "clobber" means "overwrite with garbage" for 
all the beasts we have been discussing, which translates to nothing in 
the final code, but can be used by the optimizers as Thomas said.

This is a bit off-topic but clobbers model registers having their values 
changed unpredictably or by ways unknown to the compiler, in the backend 
code, or in inline assembly statements.
Here is an excerpt from rtl.texi:
> @item (clobber @var{x})
> Represents the storing or possible storing of an unpredictable,
> undescribed value into @var{x}

I Hope it helps.